A gas turbine typically includes at least one rotor assembly in which a plurality of blades/vanes, comprising airfoils radially extending from platforms, are circumferentially fitted and distributed around a rotor disk. During operation, centrifugal forces generate circumferential rim stress in the rotating blades. As for vanes, gas pressure and vibration may also generate stress. These stresses can concentrate at the transition between the platform and the airfoil. This stress concentration can be minimized by fillets at the platform/airfoil connection portion. Adequate stress relief can however only be achieved with an adequately sized and shaped fillet.
Generally, it is desirable to reduce the size of the platform, or alternatively avoid reworking of a blade/vane when it is discovered in final design stages that there is insufficient space between the airfoil and platform edge to enable tangentially joining the fillet to the platform surface. It is therefore desirable to provide a compound fillet consisting of multiple sections of curves that requires less platform surface space without compromising mechanical integrity.
A compound fillet for a turbine blade is disclosed in EP2184442A1, which covers an airfoil to platform join and is configured to comprise a first arc and a second arc. The first arc has a first end tangential to the airfoil surface. The second arc having a first end tangentially adjoins the second end of the first arc and a second end adjoins the plat form surface. The radius of the first arc is larger than the radius of the second arc. Furthermore, another compound fillet also is disclosed in this reference, which comprises a first arc and a second arc wherein the second arc adjoins non-tangentially the platform surface.
A kind of transition between a surface of a blade/vane airfoil and a platform at an end of the airfoil is disclosed in GB2353826A, which comprises at least two curves of different radii, the radius of the curve nearest the surface of the airfoil being larger than the radius of the curve nearest to the platform. The transition may comprise two curves of different radii separated by a straight line section, or it may form a section of an ellipse.
A cooled moving blade for a gas turbine is disclosed in U.S. Pat. No. 6,190,128 B1, which has a base portion of a profile formed by an elliptically curved surface and a rectilinear surface portion, wherein the rectilinear surface portion is provided at a hub portion of the blade where thermal stress is large.
Even though different compound fillets are proposed in the above mentioned publications, there exists much more space to develop in respect of optimizing the parameters of compound fillet in order to improve the stress relief capacity of the blade/vane, hence improve the working life of the blade or vane.